Historically inorganic pigments, such as hydrated or anhydrous siliceous minerals including metal silicates, sodium alumino silicates, hydrated aluminum silicates, e.g., clays, have found increasing uses in many industries. For example, such pigments are employed in paper coating compositions, as fillers for paper, paints, inks, etc., and as reinforcing pigments or fillers in elastomers and other polymeric materials. In addition to being used as fillers, inorganic pigments are often used as color-imparting fillers in papers, paints, inks, etc. and as reinforcing pigments or fillers in elastomers and other polymeric materials.
In the process of dying pigments, it is important to insolubilize or otherwise fix dyes onto the pigment particles. In this regard, it is often necessary to treat pigment particles in such a manner so as to improve the affinity of the pigment particles to the dyestuffs utilized.
Several methods of improving the affinity of various pigment particles to dyestuffs have been reported. U.S. Pat. No. 3,834,924 to Grillo teaches a process for manufacturing surface modified inorganic pigments. The process according to this patent includes adding amino organosilanes to an aqueous dispersion of an inorganic pigment whereby the inorganic pigment is contacted with the amino organo silane and reacts so that the surface of the inorganic pigment is modified in such a manner to increase the affinity for dyes.
U.S. Pat. No. 3,545,909 to Gagliardi teaches amino-alkyl silicone coloring assistances that may be applied in the form of a pre-treatment to condition inorganic pigment substrates for a subsequent coloring operation. This pre-treatment with the aminoalkyl silicone coloring assistance induces dyeable sites onto normally non-affinitive materials so that they may accept conventional anionic type organic dyes.
U.S. Pat. No. 4,084,983 to Bernhard et al teaches a process for producing colored lustrous pigments in which a firmly adhering coating of aluminum hydroxide is first applied to pigment substrates. The aluminum ions contained in the coating are then reacted in order to form a firmly adhering dyestuff layer. In this manner, the substrate is coated with a starting material which is convertible into a colored layer.
U.S. Pat. No. 4,543,128 to Troesche et al teaches a process for dying inorganic pigments that can be used as fillers. According to this patent, pigments are dyed with polycationic dyestuffs in a process that involves providing a colored composition of an aqueous paste for dispersion of the white pigment with a water-soluble polycationic dye. An optional, conventional, fixing agent may be used in this process.
U.S. Pat. No. 4,566,908 to Nakatani et al teaches a process for producing an azoic pigment including a silica core with a coating of amino or polyazoic dye chemically bound to the surface of the silica core through an amino silane coupling agent.
While previous efforts had focused on various methods to fix diverse dyestuffs onto selective substrates, there is a long-felt need for developing methods whereby safer organic dyes could replace currently used inorganic pigments, many of which are based upon heavy metals and pose potential health problems both in their processing and use. In this regard, solvent dyes have been developed to overcome associated health concerns in many rubber and plastics applications. However, solvent dyes are usually expensive, may involve volatile and/or toxic solvent vehicles during use, are often hard to disperse, may migrate and rub off, and sometimes degrade the rubber and plastic materials into which they are incorporated.
Water-soluble, organic dyes have the potential of addressing the problems of both inorganic pigments and solvent dyes in many applications. Nevertheless, methods for stabilizing water-soluble, organic dyes, particularly dyes that can be used to replace currently used heavy metal-based pigments are lacking. The present invention provides a method for coloring inorganic pigments substrates which is an improvement over prior known methods. The water-soluble, inorganic dyes are thereby rendered functional in a variety of media and circumstances of use where advantage could not otherwise be taken of their desirable features.